BEIRUT — The Syrian regime is expanding its use of widely banned cluster bombs, an international human rights group said Saturday as the deadlocked conflict entered its third year.

In new violence, rebels detonated a powerful car bomb outside a high-rise building in the eastern city of Deir el-Zour, setting off clashes with regime troops, state TV and activists said.

The blast came a day after Syrians marked the second anniversary of the start of their uprising against President Bashar Assad. The rebellion began with largely peaceful protests, but when the regime cracked down on demonstrators, the unrest evolved into an insurgency and then a civil war.

In recent months, the Assad regime has escalated airstrikes and artillery attacks on rebel-held areas in the north and east of the country, rights groups have said.

On Saturday, the New York-based Human Rights Watch said Syrian forces have dropped at least 156 cluster bombs in 119 locations across the country in the past six months, causing mounting civilian casualties.

The regime denied that it is using cluster bombs, but the report said that two strikes in the past two weeks killed 11 civilians, including two women and five children.

Cluster bombs open in flight, scattering smaller bomblets. They pose a threat to civilians long afterwards since many don’t explode immediately. Many countries have banned their use.

Human Rights Watch said it based its findings on field investigations and analysis of more than 450 amateur videos.

A senior Syrian government official on Saturday rejected the report, saying many amateur videos were suspect. He spoke on condition of anonymity because he was not authorized to make official statements to the media.

The fighting in Syria has killed some 70,000 people and displaced 4 million of the country’s 22 million people, according to U.N. estimates.

The conflict remains deadlocked, despite some recent military gains by the rebels.

On Saturday, rebels in Deir el-Zour detonated a car rigged with more than two tons of explosives next to the tallest building in the city, known as the Insurance Building, state TV said.

The TV said rebels entered the building after the blast but were pushed out by government forces. It was not immediately known whether anyone was wounded or killed.

The Britain-based Syrian Observatory for Human Rights, an activist group, also reported clashes between rebels and regime troops following the explosion. Regime forces also shelled several areas of the city, the group said.

In an amateur video said to be showing Deir el-Zour, heavy gunfire was heard in the background and a cloud of smoke was visible.

The Observatory also said at least 12 rebel fighters were killed in clashes near a cement factory in the northern city of Aleppo and that five people were killed when a shell exploded in the Damascus neighborhood of Qaboun.

Also Saturday, the head of Syria’s leading opposition group issued an anniversary message to Syrians, saying that the uprising has “has taken a long time.”

The opposition recognizes March 15, 2011 as the start of the uprising.

In a video posted on his Facebook page, Mouaz al-Khatib, head of the Syrian Opposition Coalition, congratulated the town of Yabrud, north of Damascus, for creating a civil council to run its affairs.

“Our people are great, our people are civilized and they don’t need gangs to rule them,” al-Khatib said, sitting in front of a Syrian flag and cracking a rare smile. “They just need to breathe a little bit of the air of freedom and they’ll create as they have created in all places.”

The videos appeared authentic and corresponded with other reporting by The Associated Press.

Late Friday, rebel fighters from the al-Qaida-linked group Jabhat al-Nusra and other Islamist factions seized a military base and munitions depot in the town of Khan Touman in the northern province of Aleppo, the Observatory said.

It quoted witnesses as saying rebel fighters drove off with truckloads of ammunitions and weapons. The Khan Touman base is only a few kilometers (miles) from a military engineering academy that is considered a key government stronghold in the province, the Observatory said.

Despite rebel advances, Assad has been digging in, particularly in the densely populated western part of the country. He has armed and mobilized loyalists, and repelled rebel attacks on his seat of power, the capital, Damascus.

The rebels have appealed to the West for military aid, including anti-aircraft weapons, to help them break the stalemate.

On Friday, a European Union summit heard an appeal by Britain and France to lift the EU ban on arming the rebels.

The 27 national leaders were unable to reach a consensus and asked their foreign ministers, who will meet late next week in Dublin, to try to hash out a common position.

Samir Nashar, a member of the Syrian National Coalition, the main opposition group in exile, said he hoped France and Britain would defy the EU if the embargo remains in place.

“I prefer that there is a consensus and a joint resolution,” he said Friday in Istanbul. “But if there’s no consensus, I still think France and Britain will act unilaterally.”

The French foreign minister suggested earlier this week that his country might arm the rebels even if the EU disagrees.